Pulmonary hypertension, right ventricular hypertrophy and VQ mismatch contribute to the severity of disease in some infants who develop severe bronchopulmonary dysplasia (BPD) requiring prolonged mechanical ventilation. The aim of this study is to determine whether pulmonary hypertension, right ventricular hypertrophy and VQ mismatch in these infants are responsive to nitric oxide (NO). Secondary aims are to determine if the use of NO at 5 to 20ppm will result in improved oxygenation, while identifying the minimal dose of NO at which these effects can be seen achieving acceptable arterial blood gases at lower ventilator settings and lowered FiO2. Toxicity of NO in these patients will also be examined.